A nanowire refers to a wire having a diameter typically in the range of about one nanometer (nm) to about 500 nm. Nanowires are solid, and can have amorphous structure, graphite like structure, or herringbone structure. The nanowires are periodic only along their axis, and can therefore assume any energetically favorable order in other planes, resulting in a lack of crystalline order.
Nanowires are typically fabricated from a metal or a semiconductor material, and some of the electronic and optical properties of the metal or semiconductor materials are different than the same properties of the same materials in larger sizes. For example, metallic wires having a diameter of 100 nm or less display quantum conduction phenomena, such as the survival of phase information of conduction electrons and the obviousness of the electron wave interference effect. Semiconductor or metal nanowires have attracted considerable attention because of their potential applications in mesoscopic research, the development of nanodevices, for use as gas sensors and field emitters, and the potential application of large surface area structures. For example, U.S. Pat. No. 5,973,444 to Xu et al. discloses carbon fiber-based field emission devices, where carbon fiber emitters are grown and retained on a catalytic metal film as part of the device. Xu et al. disclose that the fibers forming part of the device may be grown in the presence of a magnetic or electric field, as the fields assist in growing straighter fibers.
One technique for fabricating quantum wires utilizes a micro lithographic process followed by metalorganic chemical vapor deposition (MOCVD). This technique may be used to generate a single quantum wire or a row of gallium arsenide (GaAs) quantum wires embedded within a bulk aluminum arsenide (AlAs) substrate. One problem with this technique, however, is that microlithographic processes and MOCVD have been limited to GaAs and related materials. Moreover, this technique does not result in a degree of size uniformity of the wires suitable for practical applications.
Another method of fabricating nanowire systems involves using a porous substrate as a template and filling naturally occurring arrays of nanochannels or pores in the substrate with a material of interest. However, it is difficult to generate relatively long continuous wires having relatively small diameters because as the pore diameters become small, the pores tend to branch and merge, and because of problems associated with filling long pores having small diameters with a desired material.
U.S. Pat. No. 6,838,720 to Krieger et al. discloses a memory device with active passive layers. The ions move from the passive layer to an active layer to form a nanowire feature. The organic layer may be phthalocyanine, but the synthesis of nanowires is not provided.
Harutyunyan et al. Appl. Phys. Lett. 82: 4794-4796 (2003) discloses pyrolysis of a metalorganic precursor for self-assembly of carbon nanotubes. Unlike nanowires, carbon nanotubes are hexagonal networks of carbon atoms forming hollow, seamless tubes with each end capped with half of a fullerene molecule. They were first reported in 1991 by Sumio Iijima who produced multi-layer concentric tubes or multi-walled carbon nanotubes by evaporating carbon in an arc discharge. Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Journet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al. Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274: 1701 (1996)). These methods are not suitable for the production of nanowires.
Thus, there is a need for methods for synthesizing metal nanowires, and for the synthesis of metal nanowires at preselected locations on a substrate. Preferably, the method allows for growth of a controlled number of metal nanowires at preselected locations on a substrate.